1. Field of the Invention
The present invention relates to a synchronous signal detection apparatus for synchronously detecting a predetermined frequency component of a signal to be measured.
2. Related Background Art
Generally, a noise intensity depends on the range of a frequency band. The influence of noise can be reduced by realizing a stable bandpass filter for selecting frequencies only close to a frequency to be measured. As an excellent system therefore, a lock-in amplifier is well-known.
FIG. 1 is a block diagram snowing the arrangement of a typical conventional lock-in amplifier. As shown in FIG. 1, in this apparatus, a light beam to be measured is received by a photodetector 910 constituted by an avalanche photodiode, and converted into an electrical signal. This electrical signal is amplified by an amplifier 920. A signal is output from an oscillator 930, and its phase is adjusted by a phase adjustment circuit 940. This signal having a frequency to be measured and the signal output from the amplifier are input to a phase detection circuit 950, and their phases are detected. Thereafter, a signal is extracted in accordance with a frequency region close to the frequency to be measured, and output via a low-pass filter 960.